Atomic force microscope

ABSTRACT

This atomic force microscope has a probe for surface analysis of a sample (E), comprising a support body and an elastically deformable strip linked to the body, the strip being provided with a tip designed to come into contact with the sample (E) to be analysed. The microscope also has a mechanism for relative displacement of the analysis probe with respect to the surface of the sample (E), a detector for determining the position of the strip, and elements for vibrating the strip. These means for vibrating the strip include elements for conduction of electricity along a continuous path forming a loop, an alternating-current generator, and a magnetic-field source designed to set up a magnetic field ({right arrow over (B)}) in the region of the strip of the analysis probe.

[0001] The present invention relates to an atomic force microscope, ofthe type having a probe for surface analysis of a sample, comprising asupport body and an elastically deformable strip linked to the body, thestrip being provided with a tip designed to come into contact with thesample to be analysed; a mechanism for relative displacement of theanalysis probe with respect to the surface of the sample; a detector fordetermining the position of the strip; and means for vibrating thestrip.

[0002] In an atomic force microscope, the probe for analysis of thesurface and the sample are moved relative to one another along lines.

[0003] Such a probe has an elastically deformable strip carried at theend of a support body. This strip is commonly referred to by the Englishterm “cantilever”. At its free end, the strip has a tip designed toenter into contact with the surface of the sample to be analysed. Thebending of the strip is measured, in particular by optical means, inorder to determine the effects of the mechanical interaction between thesurface being studied and the tip disposed at the end of the strip.

[0004] It is known, in order to reduce damage to the surface of thesample to be analysed, to vibrate the strip in a direction perpendicularto the plane of the sample, so that the tip taps the surface of thesample. The interactions of the tip with the surface generate changes inthe amplitude response or phase response of the strip, and these provideaccess to the surface properties of the sample, such as topography orelasticity. This analysis mode is generally referred to by the term ACmode.

[0005] In order to vibrate the strip, it is known, in particular fromthe document WO-99/06793, to add a particle or a deposit of a magneticmaterial on the top of the strip, and to create an oscillating magneticfield in the space where the strip is displaced. This magnetic field iscreated, for example, by a coil through which an alternating currentflows. This way of vibrating the strip is advantageous in comparisonwith conventional methods in which the vibration is inducedmechanically, because it eliminates the resonances due to thismechanical system. This is particularly borne out for use in an aqueousmedium.

[0006] The homogeneous deposition of a magnetic material on the strip isa difficult operation. Furthermore, this material changes the mechanicalcharacteristics of the strip.

[0007] It is an object of the invention to provide an atomic forcemicroscope, and a surface-analysis probe for this microscope, which canoperate satisfactorily in a mode in which the strip is vibrated and inwhich the strip can be fabricated easily.

[0008] To that end, the invention relates to a microscope of the typealready mentioned, characterized in that the means for vibrating thestrip have, on the strip, means for conduction of electricity along acontinuous path forming a loop, which electrical-conduction means aresecured to the strip, the support body being provided with two dividedconductive sections extending the loop, an alternating-current generatorconnected to the divided conductive sections of the analysis probe, anda magnetic-field source designed to set up a substantially homogeneousmagnetic field in the region of the strip of the analysis probe.

[0009] The invention also relates to a microscope of the type alreadymentioned, characterized in that the detector has, on the strip, meansfor conduction of electricity along a continuous path forming a loop,which electrical-conduction means are secured to the strip, amagnetic-field source designed to set up a substantially homogeneousmagnetic field in the region of the strip of the analysis probe, andmeans for analysis of currents induced by the magnetic field in theloop.

[0010] One or other of these microscopes according to the invention mayhave, in any technically feasible combination, one or more of thecharacteristics described in claims 2 and 4 to 12.

[0011] The invention will be, understood more clearly on reading thefollowing description, which is provided by way of example and is givenwith reference to the drawings, in which:

[0012]FIG. 1 is a diagrammatic view of the surface-analysis probearranged in a microscope according to the invention;

[0013]FIG. 2 is a view showing the change in the amplitude of thevibrations of the elastic strip of an exemplary embodiment of ananalysis probe in the presence of an excitation current whose frequencyvaries from 0 to 40 kHz; and

[0014]FIG. 3 is an image of a sample obtained with a microscope using aprobe according to the invention; and

[0015]FIGS. 4 and 5 are views similar to the one in FIG. 1, eachillustrating an embodiment of a variant of a microscope according to theinvention.

[0016] The atomic force microscope 10, only the main elements of whichare represented in FIG. 1, has a probe 12 for analysis of the surface ofa sample E. The microscope furthermore has a movable table 14 forsupporting the sample. This table 14 can be moved, in three orthogonaldirections, relative to the analysis probe 12 under the action ofdisplacement means 16 of any suitable type. These means produce relativedisplacement of the probe 12 with respect to the sample E substantiallyin the plane of the surface of the sample E. They are designed to makeit possible to scan the surface of the sample with the probe 12.

[0017] The microscope 10 furthermore has a permanent magnet 18 which ispreferably arranged under the movable table 14. This permanent magnet 18is capable of creating a permanent magnetic field {right arrow over (B)}in the region of observation of the sample. The magnet is selected sothat the field {right arrow over (B)} is set up substantiallyhomogeneously in the region where the analysis, end of the probe isdisplaced. This field {right arrow over (B)} is substantiallyperpendicular to the surface of the sample, i.e. to the direction inwhich the sample is scanned by the probe.

[0018] The analysis probe 12 has a support body 20 which permitsmechanical linkage of the probe to the structure of the atomic forcemicroscope and, more precisely, to the displacement means 16 integratedtherein.

[0019] The support body 20 has a parallelepipedal overall shape and isessentially made of glass or “pyrex”. One side face of the support body20 is extended by an elastically deformable strip 22. This strip isobtained by subsequent deposition on the body of silicon nitride Si₃N₄.The strip 22 has a small thickness which is very much less than that ofthe body, hence permitting its elastic deformation. Its thickness is,for example, 1 μm. The length of the strip is, for example, 200 μm.

[0020] The strip 22 is essentially flat and extends parallel to thesurface of the body 0.20. The strip is secured to the body 20 only atone end. It is hence cantilevered.

[0021] The analysis probe is carried in the microscope by a supportwhich ensures that the strip 22 is slightly inclined relative to thesurface of the sample E. The normal to the strip 22 is, for example,inclined by about 100 relative to the normal to the overall plane of thesample E. The normal to the plane 22 hence defines an angle of about 10°with the magnetic field {right arrow over (B)}.

[0022] At its free end, the strip 22 carries a tip 24 designed to enterinto contact, in an aqueous medium, with the sample E to be analysed.The tip extends perpendicularly to the plane of the strip 22. The radiusof curvature of the tip 24 is about 30 nm at its extreme end.

[0023] The strip 22 advantageously has two coplanar branches 26A, 26Bwhich converge towards each other. These two branches have distinctroots, where they are linked to the support body 20. Their other endsare linked to each other by a junction bend, where the tip 24 iscarried.

[0024] According to the invention, the strip 22 is provided with meansfor conduction of electricity along a continuous path forming a loop 27which extends over the essential part of the length of the strip. Thisloop is secured to the said strip.

[0025] In the embodiment which is represented, theseelectrical-conduction means are formed by a conductive coating whichcovers at least one of the faces of the strip, so as to create aconductive path. This coating is advantageously a metal coating,preferably consisting of gold. Its thickness is between 5 nm and 500 nmand is advantageously substantially equal to 50 nm.

[0026] The loop 27 formed by the metal coating extends along the twobranches 26A, 26B. It is therefore formed by a single turn which is openin the region of the support body 20.

[0027] This loop is extended by divided conductive sections 30A, 30Bdisposed on the surface of the support body 20. These conductivesections are formed by a conductive coating extending over the mainsurface of the body, and over its side surface to which the strip 22 isconnected. Advantageously, the conductive coating extending over thesupport body is of the same nature as the coating applied-to the strip.They are preferably formed simultaneously and constitute a singlecoating extending both over the strip and over the support body.

[0028] The sections 30A and 30B are separated from each other by aninsulating area 32 free of conductive coating.

[0029] According to the invention, an alternating electric-currentgenerator 34 is connected between the two conductive areas 30A and 30B.To that end, conductive pads forming the terminals of the generator 34are held applied to the conductive sections 30A and 30B, the latterhence forming, in the region where the conductive pads are applied,means for connection of the loop 27 to the alternating-current generator34.

[0030] The electric current which is delivered by the generator andflows through the loop 27 is preferably sinusoidal and its strength isbetween 0.01 and 100 mA.

[0031] Lastly, the atomic force microscope has a light source 50 whichilluminates the surface carrying the metal coating of the strip 22, aswell as a receptor 52 for detecting the position of the reflected beam.The emitter 50 and the receptor 52 are designed, as is known per se, todetermine the change in the deflection of the elastic strip 22.

[0032] The atomic force microscope operates in the following way.

[0033] During the analysis of the surface of a sample E, thealternating-current generator 34 is turned on, hence causing anoscillating electric current to flow through the loop 27 formed by theconductive coating carried by the strip 22.

[0034] The flow of the alternating electric current through this loopcreates a variable magnetic moment {right arrow over (M)}. At eachinstant, the presence of the magnetic field {right arrow over (B)}generated by the permanent magnet 18 imposes stresses on the strip 22,the magnetic moment {right arrow over (M)} thereof tending to align withthe magnetic field {right arrow over (B)}. In other words, the currentloop formed on the strip is subjected to Laplace forces which aretransmitted to the strip 22, the latter being mechanically secured tothe loop. The stresses applied to the strip result in continualdeformation of the latter, as a function of the change in the magneticmoment {right arrow over (M)}.

[0035] Because of the periodic reversal of the direction of the currentset up in the loop by the alternating-current generator 34, the magneticmoment reverses at each half-period of the generator, hence creatingoscillation of the strip 22 and, in particular, of its end carrying thetip 24.

[0036] Advantageously, the frequency of the alternating-currentgenerator 34 is set to the resonant frequency of the strip 22, or closeto this frequency.

[0037]FIG. 2 represents the vibration amplitude of the strip 22 immersedin water, for a current of the order of 1 mA flowing through the loop27, as a function of frequency.

[0038] In this figure, the frequency in hertz is given on the abscissa,whereas the amplitude of the oscillations in nanometres is representedon the ordinate.

[0039] The sinusoidal current flowing through the turn isfrequency-modulated between 0 and 40 kHz and its peak strength is about1 mA. The well-defined amplitude maximum observed at about 4 kHzcorresponds to the natural vibration frequency of the strip, i.e. to itsresonant frequency.

[0040] The strip which is used has a stiffness constant of the order of50 mN/m. The current loop was produced by depositing a 50 nm gold filmon the lower face of the strip, i.e. the face from which the tipemerges.

[0041] The very large amplitude observed for the vibrations iscommensurately higher if the frequency of the generator is close to thenatural resonant frequency of the strip.

[0042]FIG. 3 represents an image of the topography of actin filamentsdeposited on the surface of a glass slide, the image having been takenwith the proposed probe by using a suitable atomic force microscope.Each grey level of this image corresponds to an ?altitude! on thesample, as indicated by the scale.

[0043] This image is taken in water using the AC mode.

[0044] The excitation of the resilient strip is generated by virtue of asinusoidal current flowing through the loop carried by the strip, with apeak strength of the order of 1 mA. Its frequency is set at 3.7 kHz(close to the natural frequency). The output signal obtained on thephotodetector is about 5 Vrms, corresponding to a vibration amplitude ofabout 40 nm.

[0045] With a device as described here, it will be understood that thevibration of the strip in an atomic force microscope is simplified sinceit only requires the installation of a permanent magnetic-field sourceand an alternating-current generator. Furthermore, the placement of aconductive coating on the surface of the strip 22 is a well-masteredtechnique which is commonly used in the semiconductor industry, wherethe deposition of a metal coating is a standard operation.

[0046] In an alternative embodiment of the microscope, the position ofthe magnet 18 creating the magnetic field {right arrow over (B)} ismodified. In particular, the magnet is not placed with its axissubstantially perpendicular to the surface of the sample to be analysed.Instead, the magnet is disposed in such a way as to produce a magneticfield whose direction extends parallel to the surface of the sample tobe analysed, specifically in a direction parallel to the longitudinalmid-axis of the strip.

[0047] Under these conditions, when a sinusoidal current flows throughthe loop 27 carried by the strip, the strip 22 experiences twistingabout its longitudinal mid-axis. Hence, under the action of thereciprocating torsional movement of the strip, the tip 24 executesreciprocating local scanning of the surface of the sample, hence makingit possible to measure the frictional properties of the surface of thesample.

[0048] These frictional properties are deduced from analysis of themovement of the light beam reflected by the strip.

[0049] In a variant of the microscope according to the invention, thesample E to be analysed is fixed and the analysis probe 12 can be movedrelative to this sample under the action of displacement means of anyappropriate type. The operation of the microscope is then substantiallysimilar to that of the microscope in FIG. 1.

[0050] According to an alternative embodiment, which is not shown, aplurality of strips which are substantially similar to one another areorganized on the same probe. Each strip is provided with its ownelectrical-conduction loop, each loop being supplied in parallel by thealternating-current generator. Advantageously, the microscope has meansfor generating a substantially homogeneous magnetic field {right arrowover (B)} in the region where the analysis ends of these strips aredisplaced. These means are formed, for example, by a correspondinglydimensioned permanent magnet. During the displacement of the probe,since an alternating current flows through each strip, it will beunderstood that each strip vibrates independently. During the samescanning pass by the probe, the bending of each of the strips ismeasured, which makes it possible to scan the surface to be analysedmore quickly.

[0051] Furthermore, according to yet another variant, the microscopedoes not have an external strip-position detector such as the opticalsystem with light beams 50, 52 in FIG. 1.

[0052] According to a first embodiment of this variant, which isrepresented in FIG. 4, the loop 27 is not connected to analternating-current generator. However, a component 60 for mechanicalexcitation of the strip is applied to the probe. Thismechanical-excitation component is composed of, for example, anauxiliary piezoelectric mechanism which vibrates the strip 22 carryingthe loop.

[0053] The loop 27 is connected to means 70 for processing the signaldue to the induced currents. In particular, these means haveamplification means 72 associated with means 74 for measurement of thevoltage and/or the strength of the current in the loop 27, as well asmeans 76 for analysis of the amplified signals. Advantageously, themagnetic field {right arrow over (B)} produced by the source 18 is veryhigh, being more than 0.2 T and, for example, equal to 1 T.

[0054] During the relative displacement of the probe 12 above the sampleE to be analysed, since the strip 22 of the probe is mechanicallyexcited, the oscillation of the loop 27 in the magnetic field {rightarrow over (B)} gives rise, in the loop, to an electromotive force whichgenerates induced currents. This electromotive force is analysed by thesignal-processing means 70 connected to the terminals of the loop. Theanalysis of the signals amplified by the aforementioned means 76 makesit possible to determine the profile of the surface under analysis, theelectromotive force being representative of the interaction by contactbetween the measurement end of the probe and the sample surface to beanalysed.

[0055] According to another embodiment, which is represented in FIG. 5,the piezoelectric excitation mechanism is absent and, in addition to thesignal-processing means 70, the loop 27 is connected to analternating-current generator 34 as in the embodiment in FIG. 1.

[0056] During the analysis, the generator 34 causes an alternatingcurrent to flow through the loop 27. Under the action of the flow ofthis current, the strip 22 is made to vibrate, and to enter into contactwith the surface to be analysed during the relative displacement of theprobe 12.

[0057] Because of the oscillatory movement of the loop 27 in themagnetic field {right arrow over (B)}, a back electromotive force isformed in the loop and generates induced currents therein. This backelectromotive force is analysed by the signal-processing means 70, inorder to deduce therefrom the profile of the surface of the analysedsample.

[0058] In practice, it will be understood that the electromotive forceor the back electromotive force produced in the loop 27 has a voltage ofbetween 0.1 nV and 10 nV, corresponding to a strength of between 1 pAand 10 nA, for example, a voltage of about 1 nV corresponding to astrength of about 100 pA.

[0059] With such means for detection of the deformation of the strip 22,it is hence possible to determine the profile of the surface to beanalysed without employing optical means.

1. Atomic force microscope, of the type having a probe (12) for surfaceanalysis of a sample (E), comprising a support body (20) and anelastically deformable strip (22) linked to the body (20), the stripbeing provided with a tip (24) designed to come into contact with thesample (E) to be analysed; a mechanism (16) for relative displacement ofthe analysis probe (12) with respect to the surface of the sample (E); adetector (50, 52) for determining the position of the strip (22); andmeans for vibrating the strip (22), characterized in that the means forvibrating the strip have: on the strip (22), means (26A, 26B) forconduction of electricity along a continuous path forming a loop (27),which electrical-conduction means (26A, 26B) are secured to the strip(22), the support body (20) being provided with two divided conductivesections (30A, 30B) extending the loop (27), an alternating-currentgenerator (34) connected to the divided conductive sections (30A, 30B)of the analysis probe (12), and a magnetic-field source (18) designed toset up a substantially homogeneous magnetic field (B) in the region ofthe strip (22) of the analysis probe (12).
 2. Microscope according toclaim 1, characterized in that the detector has means (76) for analysisof currents induced by the magnetic field (B) in the loop (27). 3.Atomic force microscope, of the type having a probe (12) for surfaceanalysis of a sample (E), comprising a support body (20) and anelastically deformable strip (22) linked to the body (20), the stripbeing provided with a tip (24) designed to come into contact with thesample (E) to be analysed; a mechanism (16) for relative displacement ofthe analysis probe (12) with respect to the surface of the sample (E); adetector for determining the position of the strip (22); and means forvibrating the strip (22), characterized in that the detector has on thestrip (22), means (26A, 26B) for conduction of electricity along acontinuous path forming a loop (27), which electrical-conduction means(26A, 26B) are secured to the strip (22), a magnetic-field source (18)designed to set up a substantially homogeneous magnetic field ({rightarrow over (B)}) in the region of the strip (22) of the analysis probe(12) and means (76) for analysis of currents induced by the magneticfield ({right arrow over (B)}) in the loop (27).
 4. Microscope accordingto claim 3, characterized in that the means for vibrating the strip (22)have a component (60) for mechanical excitation of the strip. 5.Microscope according to claim 3, characterized in that the means forvibrating the strip (22) have an alternating-current generator (34), andin that the support body (20) of the analysis probe (12) is providedwith two divided conductive sections (30A, 30B) which extend the loop(27) and each have means for connection of the loop to the saidalternating-current generator.
 6. Microscope according to one of thepreceding claims, characterized in that the loop (27) has a single turnwhich is open in the region of the support body (20).
 7. Microscopeaccording to one of the preceding. Claims, characterized in that thestrip (22) has two branches (26A, 26B) which converge towards each otherfrom divided roots for linkage to the support body (20)as far as ajunction bend, and in that the continuous path forming the loop (27)extends along the length of the two branches (26A, 26B).
 8. Microscopeaccording to one of claims 1, 2 or 5 to 7, characterized in that theelectrical-conduction means have a conductive coating which is formed onone face of the strip (22) and extends over the support body (20) toform the two divided conductive sections (30A, 30B), the latter beingseparated by a coating free area (32) of the body.
 9. Microscopeaccording to one of the preceding claims, characterized in that themagnetic-field source (18) is designed to set up a permanent magneticfield ({right arrow over (B)}) in the region of the strip (22). 10.Microscope according to one of claims 1, 2 or 5 to 9, characterized inthat the alternating-current generator (34) has means for controllingthe frequency of the current, which are designed to bring the latter toa frequency substantially equal to the resonant frequency of the strip(22).
 11. Microscope according to one of the preceding claims,characterized in that the magnetic-field source (18) is arranged to setup a magnetic field (B) whose direction extends substantiallytransversely to the plane of the strip (22).
 12. Microscope according toone of the preceding claims, characterized in that the magnetic-fieldsource is arranged to set up a magnetic field whose direction extendssubstantially along the longitudinal mid-axis of the strip.